That's a bug, but not like you think. The bug is that you're not supposed to be able to equip melee and ranged at the same time. It's tragic in a 40k game, especially one about the inquisition, but unfortunately it's intended. They said they'd implement it if enough people want it so make some noise about it.

Indeed. The hardest part would be to make these combos work is the balance, as they would be always too weak in proportion to other, more viable weapon combinations.

Here's a more specific example. I'm new to the game, not to mention I haven't tried dual wielding yet, so I don't know exactly how dual wielding damage works. For the sake of example, let's say the difference is hypothetically 50% between ranged/melee and melee/melee, and the difference is 40% between ranged/melee and ranged/ranged. Just make equipping a ranged/melee combination give you a +45% damage buff for as long as you're wielding a ranged/melee weapon combination.